STAT5a (UniProt/Swiss-Prot entry P42229) is a member of the STAT family of transcription factors. These transcription factors are activated by phosphorylation in response to cytokines and other growth factors. Once phosphorylated the proteins are translocated from the cytoplasm to the nucleus were they activate there target genes. As such STAT proteins act as both signal transducers and transcriptional activators. STAT5 regulates the expression of milk proteins during lactation Further, STAT5 is associated with different tumors and activation of STAT5 in myeloma and lymphoma has been shown to be essential for tumorigenesis.